23,527 research outputs found
An underground cosmic ray muon telescope for observation of cosmic ray anisotropy
A telescope housed in a tunnel laboratory has an overburden of 573 hg cm(-2) and is located under the center of a saddle-shaped landscape. It is composed of triple layers of proportional counters, each layer of area approx. 4m x 2m and their separation 0.5m. Events are selected by triple coincidence and software track identification. The telescope is in operation for over a year and the overall count rate is 1280 hr(-1). The structure and operation of the system is reported
Statistical Modelling of Information Sharing: Community, Membership and Content
File-sharing systems, like many online and traditional information sharing
communities (e.g. newsgroups, BBS, forums, interest clubs), are dynamical
systems in nature. As peers get in and out of the system, the information
content made available by the prevailing membership varies continually in
amount as well as composition, which in turn affects all peers' join/leave
decisions. As a result, the dynamics of membership and information content are
strongly coupled, suggesting interesting issues about growth, sustenance and
stability.
In this paper, we propose to study such communities with a simple statistical
model of an information sharing club. Carrying their private payloads of
information goods as potential supply to the club, peers join or leave on the
basis of whether the information they demand is currently available.
Information goods are chunked and typed, as in a file sharing system where
peers contribute different files, or a forum where messages are grouped by
topics or threads. Peers' demand and supply are then characterized by
statistical distributions over the type domain.
This model reveals interesting critical behaviour with multiple equilibria. A
sharp growth threshold is derived: the club may grow towards a sustainable
equilibrium only if the value of an order parameter is above the threshold, or
shrink to emptiness otherwise. The order parameter is composite and comprises
the peer population size, the level of their contributed supply, the club's
efficiency in information search, the spread of supply and demand over the type
domain, as well as the goodness of match between them.Comment: accepted in International Symposium on Computer Performance,
Modeling, Measurements and Evaluation, Juan-les-Pins, France, October-200
Fitting Pulsar Wind Tori. II. Error Analysis and Applications
We have applied the torus fitting procedure described in Ng & Romani (2004)
to PWNe observations in the Chandra data archive. This study provides
quantitative measurement of the PWN geometry and we characterize the
uncertainties in the fits, with statistical errors coming from the fit
uncertainties and systematic errors estimated by varying the assumed fitting
model. The symmetry axis of the PWN are generally well determined, and
highly model-independent. We often derive a robust value for the spin
inclination . We briefly discuss the utility of these results in
comparison with new radio and high energy pulse measurementsComment: 15 pages, 3 figures, ApJ in pres
From computation to black holes and space-time foam
We show that quantum mechanics and general relativity limit the speed
of a simple computer (such as a black hole) and its memory space
to \tilde{\nu}^2 I^{-1} \lsim t_P^{-2}, where is the Planck time.
We also show that the life-time of a simple clock and its precision are
similarly limited. These bounds and the holographic bound originate from the
same physics that governs the quantum fluctuations of space-time. We further
show that these physical bounds are realized for black holes, yielding the
correct Hawking black hole lifetime, and that space-time undergoes much larger
quantum fluctuations than conventional wisdom claims -- almost within range of
detection with modern gravitational-wave interferometers.Comment: A misidentification of computer speeds is corrected. Our results for
black hole computation now agree with those given by S. Lloyd. All other
conclusions remain unchange
Exchange rate regimes and the twin economies of Hong Kong and Singapore
Based on a small, open-economy IS-LM prototype model, this paper examines the sources of macroeconomic instabilities in Hong Kong and Singapore operating under two similar cu訂ency board arrangements (CBAs). The empirical findings suggest that in general both extemal and intemal factors contribute to the macroeconomic volatilities observed in the two economies. Interestingly, whilst in Hong Kong interest rate is the single most important factor accounting for the variation in real GDP, price level and money supply, in most cases in Singapore the volatilities of these three macro variables cannot be attributed to a significant single facto r. Interest rate in both Hong Kong and Singapore moves in tandem with that of the US in the long run. In the short run, the US interest rate has both direct and indirect impacts on the two economies. Due to the high openness, international prices also affect domestic demands and prices in Hong Kong and Singapore. In addition, macroeconomic
volatilities in Hong Kong and Singapore are also attributable to the shocks in their domestic demand, though the relative magnitude of impact differs. Finally, there is evidence of a trade-off between exchange rate and interest rate targeting for the stability of money supply in Singapore. Our findings provide a useful framework for future research on the financial and monetary transmission mechanisms in the twin economies of Hong Kong and Singapore
Precise time-matching in chimpanzee allogrooming does not occur after a short delay
Allogrooming is a key aspect of chimpanzee sociality and many studies have investigated the role of reciprocity in a biological market. One theoretical form of reciprocity is time-matching, where payback consists of an equal duration of effort (e.g. twenty seconds of grooming repaid with twenty seconds of grooming). Here, we report a study of allogrooming in a group of twenty-six captive chimpanzees (Chester Zoo, UK), based on more than 150 hours of data. For analysis, we introduce a methodological innovation called the "Delta scale", which unidimensionally measures the accuracy of time-matching according to the extent of delay after the cessation of grooming. Delta is positive when reciprocation occurs after any non-zero delay (e.g. A grooms B and then B grooms A after a five second break) and it is negative when reciprocation begins whilst the original grooming has not yet ceased. Using a generalized linear mixed-method, we found evidence for time matched reciprocation. However, this was true only for immediate reciprocation (Delta less than zero). If there was a temporal break in grooming between two members of a dyad, then there was no evidence that chimpanzees were using new bouts to retroactively correct for time-matching imbalances from previous bouts. Our results have implications for some of the cognitive constraints that differentiate real-life reciprocation from abstract theoretical models. Furthermore, we suggest that some apparent patterns of time-matched reciprocity may arise merely due to the law of large numbers, and we introduce a statistical test which takes this into account when aggregating grooming durations over a window of time
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